The relation between myoclonic absence-like seizures (MAS) and underlying chromosome disorders was evaluated in 14 patients at three centers in Italy. Seven (50%) had chromosome anomalies, including trisomy 12p in 2 and Angelman syndrome in 4. MAS onset was at 3 years (range 4 months to 6 years), and reduced awareness and rhythmic myoclonic jerks were associated with 2- to 3-Hz generalized spike-and-wave discharges. MAS with chromosome anomalies differed slightly from typical MAE, with earlier onset, shorter absences, and no increase in muscle tone. The GABRB3 gene may play a role in the genesis of MAS in children with mental retardation and chromosome anomalies. 
COMMENT. Chromosome analysis is indicated in children with myoclonic absence seizures and mental retardation. Resistance to therapy and learning disabilities are frequent complications of the syndrome of epilepsy with myoclonic absences (see Progress in Pediatric Neurology III, PNB Publ, 1997;p57).
Cortical myoclonus studied by magnetoencephalography is reported in 6 patients, one with Lennox-Gastaut syndrome, from Kyoto University School of Medicine, Japan . Cortical myoclonus is myoclonus with preceding cortical electrical activity. Negative sharp waves at the precentral cortex are associated with myoclonus in LGS and Down syndrome, whereas surface positive activity precedes the EMG discharge in other cases. MEG permits more precise anatomical localization of cortical electomagnetic activity than does simultaneous EEG recording alone.